1. Field of the Invention
The present invention relates to a video tone control circuit of a color television receiver. More specifically, the present invention relates to a video tone control circuit for controlling a contour of a picture to be sharp or soft by emphasizing or attenuating a high frequency component in a video signal.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an outline of a conventional color television receiver constituting the background of the invention and FIG. 2 is a graph showing one example of a frequency characteristic of a video signal for the purpose of video tone control.
Now referring to FIGS. 1 and 2, a video tone control circuit will be briefly described. A broadcasting wave is received by a tuner 3 through an antenna 2 and a video intermediate frequency signal is obtained from the tuner 3. The video intermediate frequency signal is applied through a video intermediate frequency amplifier 4 to a video detecting circuit 5. The video detecting circuit 5 detects the video intermediate frequency signal and provides a video signal to a contrast circuit 7 and a color process circuit 11. The contrast circuit 7 adjusts a contrast ratio of the video signal and provides the output signal to a video tone control circuit 8. The video tone control circuit 8 serves to control the contour of a picture to be sharp and also to control the contour of the picture to be soft. The former control is referred to as a sharpness control and the latter control is referred to as a softness control. The output signal from the video tone control circuit 8 is applied to a brightness control circuit 9, whereby the brightness is adjusted. The output signal from the brightness control circuit 9 is applied through a video output circuit 10 to a color matrix circuit 12. On the other hand, the color process circuit 11 demodulates color difference signals from the video signal and provides the same to the above described color matrix circuit 12. The color matrix circuit 12 is responsive to the color difference signals and the brightness signal to provide R, G and B signals, which are applied to a cathode-ray tube 13.
Meanwhile, the above described video tone control circuit 8 has a frequency characteristic for the input video signal, which is approximately flat between 0 to 4 MHz, as shown by the curve A in FIG. 2. In order to perform a sharpness control, it is necessary to emphasize the video signal in the vicinity of the frequency of 2 MHz by say 3 to 6 dB, as shown by the curve B in FIG. 2. On the other hand, in the case of a softness control, it is necessary to control the video signal so that the same is attenuated in the vicinity of the frequency of 2 MHz by say 3 dB, as shown by the curve C in FIG. 2. More specifically, the video tone control circuit need comprise a circuit for emphasizing a high frequency component of the video signal in order to perform a sharpness control and a circuit for attenuating a high frequency component of the video signal for performing a softness control. A video tone control circuit comprising a sharpness control and a softness control has already been proposed and the same has also been implemented in an integrated circuit. However, a conventional video tone control circuit comprised an amplifying circuit for performing a sharpness control and an amplifying circuit for performing a softness control which are separately provided. Since a conventional video tone control circuit is structured such that each of the amplifying circuits for performing sharpness control and softness control, respectively, is controlled with respect to the gain thereof and accordingly a circuit configuration became unavoidably complicated. In addition, an LC filter and the like is required in order to emphasize or attenuate a high frequency component. A conventional video tone control circuit employed these components all as externally connected components of an integrated circuit. Therefore, as the number of components being externally connected of an integrated circuit increases, the number of terminals of an integrated circuit for externally connecting these components accordingly increases. As well known, an increase of the number of terminals of an integrated circuit is not much preferred.